Many lighting fixtures, portable lights and display panels use white light emitting diodes (LED's) as light sources. White LED's can be constructed from junctions of various semiconductor material to produce different color spectral-peaks (wavelength bands of significant emissions) of light. The more efficient (on a watts/lumens basis) white LED's have followed the path invented by Shuji Nakamura, and pioneered by Nichia Corp Japan. These white LED's were based on an Indium Gallium Nitride (InGaN), AlGaInP and similar GaN junctions, characterized by constituting a blue photon pump and a companion phosphor within the LED assembly, or located remotely but within the light path of the photon-pump. The emission of blue light (in the 430˜465 nm range) being partially absorbed by the phosphor particles (e.g. YAG phosphor, comprising of rare earth metal Yttrium, and Aluminium, Oxygen Y3Al5O12) and re-emitted in a lower, broad wavelength range, centered around yellow 552 nm (e.g., exhibiting the Stokes shift, down-converting higher-energy blue light photons, into longer wavelength photons). The combination of blue and broad-yellow re-emissions generates what appears to a typical human eye to be white light, by virtue of the use of complementary blue and yellow colors (e.g., the broad spectrum shown at the top right of FIG. 2).
One challenge of a Blue+YAG LED is that a blue-ish tint can be perceived from these nominally “white” LED's. This tint is exacerbated over time if the proportion of yellow produced by the YAG is reduced, or fades, or if the dominant spectral peaks shift with age. Another change causing a perceived shift away from white light can be blue emissions from the photon pump fluctuating in wavelength or intensity, for example with supply forward-voltage/current, temperature and/or duty-cycle in pulse-width modulation. By any means, producing an imbalance in the ratio of the specific blue and yellow dominant spectral wavelengths or amplitudes alters the color ratio required to produce the desired shade of white—termed the “white point.” Typically, the white-Point is expressed as either CIE standard reference Illuminant D-point (for example D65, or D50), or as a “color temperature” in degrees Kelvin (for example D65=color temperature of 6504K).
Another challenge for forming high-performance white LED's is the reliance on rare-earth metals (e.g., Yttrium), which are geographically sparse, creating a heavy dependence on a single region. Alternative phosphor arrangements based on a combination of red silicon nitrides (as per GE Corporations “KSF” phosphor) and green silicon alumina nitride (as per α and β-SiAlON phosphor from Fujikara Corp.) have emerged in the space, producing richer white through stronger, narrower red & green dominant primaries. The new phosphors come with additional challenges in that the emission characteristics are uneven, and degrade more quickly at independent aging rates. What is desirable is a solution that can produce a selected color temperature of white light through selective primaries, using existing or more readily available phosphor materials.
Organic LED's (OLED's), which are based on emissions of organic materials (instead of the Inorganic materials used in LED junctions), are rising in popularity as well due to improvements in intensity, luminosity per-mm2 and per-Watt, and are competing with LED's as another viable light source. The challenges with OLED as a light source are similar to those of LED's, and further the organic electro-luminescent or electro-phosphorescent materials tend to aging more quickly (for example dropping 10% in less than 1,000 hrs), accelerated through the action of oxidization and humidity. Additionally, the materials used for the primary colors have different efficiency and aging rates—blue in particular aging as much as twice as fast as green, which degrades 10% faster than red—each affecting the color balance as the primaries degrade differently. What is desirable is a solution that can produce a more stable selected color temperature of white light, through regulated primary emission.
Conventional lighting strategies for electronic liquid crystal displays (LCDs) provide for illumination for the display in one of three modalities: direct backlight, front lit and edge lit. A direct backlight is configured with a light source (e.g. a LED, OLED or Electroluminescence (EL) layer) positioned directly behind a pane of the display (typically a glass pane), such that illumination from the light source transmits through the pane. For a front lit system the light source is placed substantially in front of the viewing plane, typically in the front bezel along the inside-edge. Light from the front light passes into the display plane, through an optical stack, reflects off the back-reflector and then passes out again through the pixels. An edge lit backlight positions the light source at one or more edges of a device, and is designed to conserve both thickness, space, and power consumed by the device. A typical edge lit backlight has structure as depicted in FIG. 1, which illustrates a schematic overview and cross-section of an edge-lit backlight and display. The display and edge lit backlight 20 are shown in cross-section A-A. A light source 22 (e.g., an LED, OLED, CCFL, Laser or EL) is positioned along an edge of the display, and can be housed within a housing 24. The light source 22 is positioned such that light emitted is directed toward an optical stack 26 of the display, the optical stack 26 typically including a number of layers. A light guide 28 is configured to direct illumination from the light source 22 across the breadth of the display. The light directed from the light guide passes through layers of the optical stack 26, which can include, for example, a diffuser layer 30, a brightness enhancement layer 32 (e.g., brightness enhancement film (BEF)), a first polarization film 34, a liquid crystal film 36, and a second polarization film 38.
Typically, the light source 22 includes LEDs in a backlight bar (for example, a string of LEDs along a PCB), arranged along an edge of the display. The LEDs can be arranged along one, two, or all four edges of the display. As light from LEDs is directed into the light guide 28 (which is often wedge-shaped), a series of microdivots along an exiting face of the light guide 28 causes light to scatter, directing some of the light to go forward through the polarization films 34 and 38, along with the liquid crystal film 36 (the combination of polarization films 34 and 38, and the liquid crystal film 36 can be referred to as an “LC stack”). Often the light guide 28 includes a reflective surface (e.g., ESR) to reflect light from back side of the light guide wedge toward the illumination side of the display. While edge- or direct backlighting may be used for any LCD display, direct backlighting solutions do not require a light guide and tend to be more efficient at directing light energy through the optical stack to the viewer. However, direct backlight solutions are thicker and heavier and thus these configurations are typically only used for TVs and computer monitors where display thickness is not as critical. Edge lit configurations are often used for mobile devices, tablets, and laptops where weight and reduced thickness of the overall display is more important, and there is insufficient room for a direct lighting solution.
A principle of operation for a liquid crystal display is to sample light with an aperture comprising two linear polarizers, and a polarization-controlling liquid crystal, in the optical stack. In one LCD embodiment there is only one layer that relates to the color developed in the display, that being the color filter. That is, what is displayed on the LCD in terms of color, and the range of colors (e.g., the color gamut), is determined by the purity of the light source, and the color filter that is present. In other LCD embodiments, for example those using color-field-sequential displays, there is no color filter, the LC layer still operates as an aperture, however the light source is selectively alternated to display each of the primary colors in repeating order, in sync with the moment of displaying the pixels of the frame in that primary color. By either means the light source is therefore an important element in determining the color and quality of the image seen on an LCD display.
FIG. 2 depicts an exemplary formulation of red, green, and blue primary colors for an LCD using a white light source in a conventional backlight. The light source shown is a white LED, which is the primary light source in a modern display. In LCD display panels, this broad-spectrum white light is filtered in the display by color filters at selected central wavelengths, typically corresponding to red, green, and blue wavelengths (e.g., 640, 532, and 467 nm). The incident illumination from the light source dictates the characteristics of each of the color filters, both in the amount of absorption of incident illumination required, and in the breadth of the color filter (in terms of wavelengths passed). The illumination from the light source, filtered by the color filters, forms display primaries that are intended to be perceptually significant in intensity, so as to display the target color temperature of white (an expression of the hue and shade of white, based on the spectral emission characteristics at the given temperature of a black-body emitter), when used in unison, but having broad spectrums centered about the desired wavelengths.
Such light sources when used in conventional display backlights lead to relatively poor color saturation and intensity at the desired primary wavelengths of the display. The cause being the wavelength spread of the primary colors being too broad, and not concentrated sufficiently at the desired wavelengths spectrum to produce crisply defined tri-axial points from which can be rendered a gamut of colors. The conventional color filters included in the display break up the light source spectrum into separate colors on the display (e.g. in subpixel components of red, green, and blue) by absorbing light energy not of the desired spectral range, typically expending the absorbed energy as heat. Conventional efforts to improve the color of the display have increased the role of the color filter by increasing the thickness of the color filters as band-pass filters, or by use of filter material with stronger absorption characteristics. While this can make the range of wavelengths centered about the desired wavelengths spectrum narrower (in terms of the passband), because the color filter is an absorptive filter, a thicker filter is needed to create a narrower bandpass, which reduces further still the amount of light transmitted through the filter and ultimately through the display. Additionally there is a physical limit to the thickness which a conventional color filter can be increased in an LCD color filter before it exceeds the space available in the coating area of the optical assembly. For example, in order to achieve the Digital Cinema Initiative DCI-P3 standard for color gamut, the filter thickness required in a conventional backlight can result in an additional 50˜75% reduction of transmitted light from light source, and thereby a 2×˜3.5× increase in backlight power can be required to produce an equivalent brightness to a narrow gamut display. Moreover higher brightness requires more LED's, a wider bezel to contain more LED's, higher power, more cooling, and larger battery to maintain equivalent product operating time for portable system—all of which contributing to higher system cost, greater weight and reduced product user-friendliness. It is desirable to solve the challenge of producing wider color gamut with higher efficiency rather than simply increasing brightness and backlight power.
In LED array display panels, the individual pixels are constructed from groups of LED's. For example each pixel includes one LED for each of the primary colors of the sub-pixel. Such LED displays are typically used in wall displays or outdoor street advertising where brightness and size are required. In some embodiments each sub-pixel primary color LED contains a phosphor stimulated by a diode junction photon pump to emit in the desired primary color spectrum. In some configurations the primary color is achieved through tuning the junction material, and design of the photon-pump, to emit directly in the desired spectral range. The color spectrum output of such LED embodiments is variable due to design and operating factors over the life of the LED, such as the voltage applied at the diode junction. Alternatively, in LED's with phosphor elements, increased temperature of the phosphor causes a quenching effect that diminishes relative output and thereby the color balance between photon-pump and phosphor. The percentage duty cycle use of one primary color LED compared to other primaries alters the relative color output of each pixel group, causing a shift in white balance and color reproduction. The ability to tune operating spectral emission ranges of LED primaries to constrain at specific desired wavelengths, regardless of aging and usage characteristics, is highly desirable.
In OLED display panels of one configuration, the individual pixels are created from organic phosphor sub-pixels of each primary color (e.g. red, green, blue), where each sub-pixel color emission comes from an organic phosphor stimulated by electric potential generated in the display backplane. In OLED display panels of a secondary configuration, the pixel includes one or more organic phosphor elements producing white emission at each sub-pixel, which is then filtered by a color filter layer arranged to pass wavelengths of the desired primary colors for each sub-pixel—similar in function to the Color Filter in an LCD. In both configurations a key challenge for OLED arises as the individual sub-pixel elements age, in particular the sub-component organic phosphors age differently, for example blue phosphor tends to degrade faster than red or green. Additionally, the relative use of each sub-pixel can disproportionally accelerate the shift in emission spectrum according to the amount of relative usage. Hence color non-uniformity and even “image sticking” are common problems whereby the color emission degradation over time becomes visibly noticeable to a typical human eye. The ability to tune operating spectral emission ranges of OLED primaries and constrain emission to specific desired wavelengths, regardless of aging, and usage characteristics, is highly desirable. In the secondary OLED configuration, the spectral output of white sub-pixel OLED's is again limited by the absorption characteristics in the absorptive color filter. The color gamut is dependent on the primaries thus filtered, and the energy absorbed by the color filter, which is effectively wasted, lost in heat. Current OLED displays have reached limit of around 90˜100% of NTSC, and are not currently capable of rendering a wider color gamut, such as is needed for DCI-P3 or BT. 2020, which has been achieved with competing LCD technology using Quantum Dot particles. OLED displays have deeper blacks and better contrast ration than LCD's, and tend to have lower power in mobile applications. It would be advantageous to achieve an equally wide color gamut for OLED displays.